1. Field of the Invention
The present invention relates to a variable gain amplifying circuit which is connected to a front stage of an antenna tuning circuit and has a collector-grounded transistor or a drain-grounded FET for impedance matching at an output stage such that an input RF signal amplified in the transistor or the FET at the output stage has no distortion when the intensity of an electric field of the RF signal is strong.
2. Description of the Related Art
Conventionally, an RF signal amplifying circuit, which is connected to a front stage of an antenna tuning circuit so as to amplify a received television signal in a television tuner, has been known. The RF signal amplifying circuit has an input stage to which an input adjusting unit is connected so as to correspond to the intensity of an electric-field of an input RF signal, the impedance of the input adjusting unit being adjusted corresponding to the intensity of an electric field of an input RF signal.
FIG. 3 is a circuit diagram showing an example of the configuration of the above-mentioned RF signal amplifying circuit in the related art, together with a subsequent circuit, which is disclosed in JP-A-11-220342.
Referring to FIG. 3, an RF signal amplifying circuit 50 includes an amplifying transistor 51, an input adjusting transistor 52, a collector load inductor 53, an emitter resistor 54, a bypass capacitor 55, a coupling capacitor 56, base bias resistors 57 and 58, a coupling capacitor 59, an emitter resistor 60, a bypass capacitor 61, a buffer resistor 62, a switch 63, a bias power supply 64, an antenna input stage 65, a signal output stage 66, and a power supply terminal 67, all of which are connected as shown in FIG. 3. In addition, a subsequent circuit connected to the signal output stage 66 of the RF signal amplifying circuit 50 includes a VHF tuning circuit (VHF TN) 74, a VHF AGC amplifying circuit (AGC AMP) 68, a UHF tuning circuit (UHF TN) 69, a UHF AGC amplifying circuit (AGC AMP) 70, buffer resistors 71 and 72, and an AGC voltage supply terminal 73, all of which are connected as shown in FIG. 3.
The operation of the above-mentioned RF signal amplifying circuit 50 is as follows.
When the intensity of the electric field of an input RF signal is weak or medium, a variable contact point 63(0) of the switch 63 is switched to one fixed terminal 63(1) and a ground voltage is supplied to a base of the input adjusting transistor 52. At this time, the input adjusting transistor 52 is cut off and drain-source impedance of the transistor 52 becomes considerably increased. Accordingly, the RF signal supplied to the antenna input stage 65 is not attenuated in the input adjusting transistor 52 to be supplied to a base of the amplifying transistor 51. In addition, the amplifying transistor 51 amplifies the RF signal supplied to the base of the transistor 51, and the amplified RF signal is output from a collector of the transistor 51 to be supplied to the subsequent circuit through the signal output stage 66.
Further, when the intensity of the electric field of the input RF signal is strong, a variable contact point 63(0) of the switch 63 is switched to the other fixed terminal 63(2) and an output bias voltage of the bias power supply 64 is supplied to a base of the input adjusting transistor 52. At this time, the input adjusting transistor 52 is turned on by supplying the bias voltage to the base and the drain-source impedance of the transistor 52 becomes smaller than before. Accordingly, the RF signal supplied to the antenna input stage 65 is slightly attenuated by the input adjusting transistor 52 to be supplied to a base of the amplifying transistor 51. In addition, the amplifying transistor 51 amplifies the RF signal supplied to the base of the transistor 51, and the amplified RF signal is output from a collector of the transistor 51 to be supplied to the subsequent circuit through the signal output stage 66.
Furthermore, in the subsequent circuit, when a VHF television signal is output from the signal output stage 66 of the RF signal amplifying circuit 50, the VHF television signal is selectively extracted from the VHF tuning circuit 74 to be supplied to the VHF AGC amplifying circuit 68. The VHF AGC amplifying circuit 68 amplifies the supplied VHF television signal with a signal gain set by an AGC voltage supplied to the AGC voltage supply terminal 73 and the amplified VHF television signal is supplied to a mixer or an intermediate frequency circuit at a subsequent stage. On the other hand, when a UHF television signal is output from the signal output stage 66 of the RF signal amplifying circuit 50, the UHF television signal is selectively extracted from the UHF tuning circuit 69 to be supplied to the UHF AGC amplifying circuit 70. The UHF AGC amplifying circuit 70 amplifies the supplied UHF television signal with the signal gain set by the AGC voltage supplied to the AGC voltage supply terminal 73 and the amplified VHF television signal is supplied to the mixer or the intermediate frequency circuit at the subsequent stage.
In this manner, in the RF signal amplifying circuit 50, when the intensity of the electric field of the input RF signal is weak or medium, the input RF signal is supplied to the amplifying transistor 51 to be amplified without being attenuated in the antenna input stage 65, but when the intensity of the electric field of the input RF signal is strong, the input RF signal is supplied to the amplifying transistor 51 to be amplified after being slightly attenuated in the antenna input stage 65. Accordingly, even when the intensity of the electric field of the input RF signal is strong, the amplifying transistor 51 is saturated, and thus the RF signal has no distortion.
However, in the above RF signal amplifying circuit 50 disclosed in JP-A-11-220342, when the intensity of the electric field of the input RF signal is weak or medium, the input RF signal is supplied to the amplifying transistor 51 without being attenuated, but when the intensity of the electric field of the input RF signal is strong, the input RF signal is supplied to the amplifying transistor 51 after being slightly attenuated. Therefore, it is difficult for the amplification of the RF signal to sufficiently follow the variation of the intensity of the electric field of the RF signal, and signal distortion may occur in some cases. In addition, since the switching operation on the intensity of the electric field is manually performed, a complicated control operation for switching between the electric field intensities is required to regularly monitor the variation of the intensity of the electric field of the RF signal.